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Macroscopic model formulae describing anisotropic anchoring of nematic liquid crystals on solid substrates

Derfel G., Buczkowska M.

Scientific Bulletin. Physics / Lodz University of Technology

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2015

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Vol. 36

5--12

EN

Formulae used for description of anchoring energy of nematic liquid crystal aligned on solid substrates are reviewed. They are based on macroscopic approach considering the concepts of the easy axis and of the anchoring strength parameters. Properties of the modified formula proposed in our earlier article are illustrated by exemplary plots which allows for comparison with other formulae. The modified formula describes the dependence of energy on the azimuthal and polar deviation of director from the easy axis and is valid qualitatively for deviation of any magnitude.

PL

Artykuł zawiera przegląd wzorów stosowanych do opisu energii kotwiczenia ciekłych kryształów nematycznych na powierzchniach ciał stałych. Wywodzą się one z makroskopowego podejścia wykorzystującego pojęcie osi łatwej i energetyczne parametry opisujące oddziaływanie ciekłego kryształu z podłożem. Opisano właściwości zaproponowanego we wcześniejszym artykule zmodyfikowanego wzoru, uwzględniającego anizotropię oddziaływania powierzchniowego, który ujmuje zależność energii kotwiczenia od odchylenia direktora od osi łatwej i jest słuszny jakościowo dla odchylenia o dowolnej wielkości. Zilustrowano go przykładowymi wykresami pozwalającymi na porównanie z innymi wzorami.

2

Analysis of deformations of flexoelectric homeotropic liquid crystal layers with various anchoring strengths

Buczkowska M., Derfel G.

Opto-Electronics Review

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2011

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Vol. 19, No. 1

56-60

EN

Deformations of homeotropic layers of nematic liquid crystal possessing flexoelectric properties and subjected to external dc voltage were studied numerically. The influence of the anchoring strengthWwas investigated. The calculations were performed for two types of layers, one with positive dielectric anisotropy and positive sum of flexoelectric coefficients and the other with opposite signs of these parameters. Strongly blocking electrodes were assumed. The threshold voltages for deformations were computed for several anchoring strength values and for low, moderate, and high ion contents. The director distributions were also determined. They were influenced by joined effects of anchoring strength and space charge of ions. For increasing ion contents and increasing anchoring strength, remarkable change of deformation character was noticed. In the case of negative dielectric and flexoelectric parameters, the threshold was nearly proportional to W for low anchoring strength and saturated at some high value of W. In the case of positive dielectric and flexoelectric parameters, the threshold voltage increased strongly with W. In the range of enhanced ion concentrations, the deformation arose in two voltage ranges. The increasing anchoring strength damped the deformations in the whole layer, however, this effect was minor at high ion content.

3

Numerical studies of significance of anchoring strength energy for behaviour of flexoelectric homeotropic nematic layer subjected to dc electric field

Buczkowska M., Grzelczak K.

Scientific Bulletin. Physics / Lodz University of Technology

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2009

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Vol. 30

5-18

EN

Elastic deformations of homeotropic nematic liquid crystal layers subjected to a d.c. electric field were studied numerically. The flexoelectric properties of the nematic material and the presence of ionic space charge were taken into account. The influence of anchoring strength W was investigated. The calculations were performed for layers with negative dielectric anisotropy and negative sum of flexoelectric coefficients. Strongly blocking electrodes were assumed. The threshold voltages for deformations were computed for several anchoring strength values and for low, moderate and high ion content. Its value was nearly proportional to W for low anchoring strength and saturated at some high value of W. The director distributions were also determined. This task was complicated by the presence of space charge of ions. Remarkable change of deformation character was noticed when the ion content and anchoring strength were varied.

PL

Zbadano numerycznie wpływ energii kotwiczenia ciekłego kryształu z podłożem na indukowane stałym polem elektrycznym deformacje homeotropowej warstwy nematyka o ujemnej anizotropii dielektrycznej i ujemnej sumie współczynników fleksoelektrycznych. Przy niskiej zawartości jonów i wysokiej energii kotwiczenia stwierdzono symetryczny rozkład direktora. W miarę wzrostu koncentracji jonów lub obniżania energii kotwiczenia deformacja stawała się asymetryczna. Wyniki pokazują, że jonowy ładunek przestrzenny wpływa znacząco na efektywność oddziaływań powierzchniowych.